1. Field of the Invention
The present invention relates generally to copolymers of vinyl monomers. More specifically, the present invention is directed to copolymers containing isobutylene type monomers and their use as flow control agents in thermosetting coating compositions.
2. Description of Related Art
It is often observed that monomers that do not readily homopolymerize are able to undergo rapid copolymerization reactions with each other. The most typical situation occurs when a strong electron donating monomer is mixed with a strong electron accepting monomer from which a regular alternating copolymer results after free radical initiation. Maleic anhydride is a widely used example of a strong electron accepting monomer. Styrene and vinyl ethers are typical examples of an electron donating monomer. Systems, such as maleic anhydride—styrene, are known to form charge transfer complexes, which tend to place the monomers in an alternating sequence prior to initiation. The application of the free radical initiator “ties” the ordered monomers together to form an alternating copolymer (Cowie, Alternating Copolymers, Plenum, New York (1985)).
U.S. Pat. No. 2,378,629 to Hanford and U.S. Pat. No. 4,151,336 to Sackmann et al. disclose that even when a moderately electron donating monomer, such as diisobutylene, is copolymerized with a strong electron acceptor monomer, such as maleic anhydride, an alternating copolymer results.
When a moderately electron donating monomer, such as isobutylene, is copolymerized with a moderately electron accepting monomer, such as an acrylic ester, poor incorporation of the electron donating monomer results. For example, free radical copolymerization of isobutylene (IB) and acrylic monomers have resulted in copolymers that contain at most 20–30% of IB and have low molecular weights because of degradative chain transfer of IB. Examples of such copolymerizations of IB are disclosed by U.S. Pat. No. 2,411,599 to Sparks et al. and U.S. Pat. No. 2,531,196 to Brubaker et al.
The ability to make copolymers of acrylic monomers and IB type monomers is desired in the art. For example, many patents express the potential for using IB-containing polymers in coating compositions. For example, U.S. Pat. No. 6,114,489 to Vicari et al. discloses a coating composition that includes a functional acrylic resin binder; a co-reactant capable of reacting with the functionality of the acrylic binder; a degasser; and a hyperbranched polyester flow and leveling agent. IB is suggested as a potential co-monomer for use in the acrylic binder as part of a long list of monomers. U.S. Pat. No. 5,552,487 to Clark et al. discloses powder coating compositions that include a copolymer having a reactive functionality and a suitable crosslinking agent capable of reaction with the reactive functionality of the copolymer. The copolymer is a made by copolymerizing functional monomers with other monomers, isobutylene being one among many listed as potential co-monomers. Although only two are referenced herein, of the many patents that express the possibility of using isobutylene-type co-monomers, none actually shows or discloses a working example of such a copolymer.
The fact that few examples of isobutylene-type monomer-containing copolymers are found is due to the generally non-reactive nature of isobutylene with acrylic and methacrylic monomers. Reactivity ratios for monomers can be calculated using the Alfrey—Price Q-e values (Robert Z. Greenley, Polymer Handbook, Fourth Edition, Brandrup, Immergut and Gulke, editors, Wiley & Sons, New York, N.Y., pp. 309–319 (1999)). The calculations may be carried out using the formulas I and II:r1=(Q1/Q2)exp{−e1(e1−e2)}  Ir2=(Q2/Q1)exp{−e2(e2−e1)}  IIwhere r1 and r2 are the respective reactivity ratios of monomers 1 and 2, and Q1 and Q2 and e1 and e2 are the respective reactivity and polarity values for the respective monomers (Odian, Principals of Polymerization, 3rd Ed., Wiley-Interscience, New York, N.Y., Chapter 6, pp. 452–467 and 489–491 (1991)). Table 1 shows the calculated reactivity ratios of selected monomers with isobutylene:
TABLE 1Monomerr1 (isobutylene)r2Methyl acrylate0.1013.67Glycidyl methacrylate0.0834.17Methacrylic acid0.0939.71
As one skilled in the art of polymer chemistry can appreciate, when r1 is near zero and r2 has a value of 10 or more, monomer 2 is reactive toward both monomers and monomer 1 is reactive toward neither monomer. In other words, it is extremely difficult to prepare copolymers having significant amounts of both monomers. It is not surprising then that few examples can be found of coating compositions that include isobutylene-type monomer-containing copolymers, because the monomers do not tend to copolymerize.
A few examples of acrylic ester or acrylonitrile copolymers made by copolymerizing with monomers such as propylene, isobutylene, and styrene, have been accomplished in the presence of Lewis acids, such as alkylaluminum halides, to give 1:1 alternating copolymers. The alternating copolymers were obtained when the concentration ratio of the Lewis acids to the acrylic esters was 0.9 and the concentration of IB was greater than the concentration of the acrylic esters (Hirooka et al., J. Polym. Sci. Polym. Chem., 11, 1281 (1973)). The metal halides vary the reactivity of the monomers by complexing with the them. The electron donor monomer—electron acceptor monomer—metal halide complex leads to alternating copolymers (Mashita et al. Polymer, Vol. 36, No. 15, pp. 2973–2982, (1995)).
Copolymers of IB and methyl acrylate (MA) have also been obtained by using ethyl aluminum sesquichloride and 2-methyl pentanoyl peroxide as an initiating system. The resulting copolymer had an alternating structure, with either low (Kuntz et al, J. Polym. Sci. Polym. Chem., 16, 1747 (1978)) or high isotacticity in the presence of EtAlCl2 (10 molar % relative to MA). (Florjanczyk et al., Makromol. Chem., 183, 1081 (1982)).
Another method for making IB copolymers with acrylic esters involved alkyl boron halide, which was found to be much more active than alkyl aluminum halides in forming alternating copolymers. The resulting copolymer was an elastomer of high tensile strength and high thermal decomposition temperature with good oil resistance, especially at elevated temperatures (Mashita et al., Polymer, 36, 2983 (1995)).
U.S. Pat. No. 5,807,937 to Matyjaszewski et al. discloses a method of making alternating copolymers of isobutylene and methyl acrylate using an atom transfer radical polymerization (ATRP) process. The method requires the use of a suitable ATRP initiator, such as 1-phenylethyl bromide, and suitable transition metal salts, such as CuBr, with a ligand, such as 2,2′-bipyridyl, to perform the complex redox initiation and propagation steps of the polymerization process.
Copolymers containing relatively high amounts (≧30 mol %) of IB and acrylic esters have only been attained by free radical polymerization when Lewis acids or ATRP initiation systems have been employed. The polymer that results from such processes requires expensive and time consuming clean up to remove the transition metal salt and/or Lewis acid residues in order to make the polymer commercially useful.
Coating compositions, liquid and powder coatings for example, are used in a wide variety of applications, including for example, the automotive, appliance, and industrial markets. Coatings are often used to provide decorative qualities and/or corrosion protection to the substrates over which they are applied. Correspondingly, applied coatings are typically required to have at least a continuous defect-free surface. The automotive industry has particularly strict requirements as to the smoothness of the coatings that are used, as is the case with automotive clear topcoat compositions.
Coating compositions typically contain a flow control agent (also referred to as a flow modifier) to improve the appearance of the cured coating. Flow control agents have surface active properties and are thought to improve the appearance of a cured coating by altering the flow and leveling of the applied coating during its cure cycle. Flow control agents containing functional groups, such as carboxylic acid groups and/or hydroxyl groups, are known and, in addition to enhancing appearance, can also improve adhesion of the coating to the substrate over which it is applied, and/or improve the adhesion or compatibility of a subsequently applied coating.
Coating compositions are typically required to provide optimum properties, such as appearance and/or corrosion resistance, at a minimum film thickness. For example, in the automotive industry clear topcoats are typically required to have cured film thickness of no greater than 50 microns (2 mils). Advantages associated with coatings applied at lower film thickness include, for example, reduced material costs and weight gain of the coated ware, which is particularly desirable in the aircraft industry. However, as the film build of an applied coating composition is decreased, the appearance of the resulting cured coating typically diminishes, for example, as evidenced by lower measured gloss values.
In addition to the application of coatings at lower film builds, investigation and development in recent years has been directed toward reducing the environmental impact of coating compositions, in particular, the associated emissions into the air of volatile organics during their use. Accordingly, interest in coatings having lower volatile organic content (VOC), for example powder coatings and high solids coatings, has been increasing. Powder coating compositions are free flowing particulate compositions that are essentially free of solvents. The appearance of powder coatings typically degrades rather precipitously with decreasing film thickness, for example, at film thickness less than 75 microns (3 mils), and, in particular, at film thickness less than 50 microns (2 mils). In the absence of solvents that can enhance the flow and leveling of an applied coating, a flow control agent is a critical component in the majority of powder coating compositions.
Copolymer compositions that contain Lewis acids and/or transition metals intermingled with the copolymer can have a number of drawbacks when used commercially, as in coatings for example. First, some Lewis acids and transition metals are toxic and have adverse environmental effects if they are leached from the copolymer and enter the environment. Second, in coating applications the Lewis acids and transition metals may lead to poor stability when exposed to UV light or simply cause the coating to discolor. In other applications the Lewis acids and transition metals may react with other ingredients in a formulation resulting in undesired properties, such as a shortened shelf-life for a given formulation.
Therefore, there is a clear and present need for copolymers containing isobutylene type monomers that are not made using and are substantially free of Lewis acids and/or transition metals.